1. Field of the Disclosure
The present invention relates generally to an optical image capture system, and more particularly to such a system having a folded optical path with reduced or eliminated secondary reflections.
2. Description of the Related Art
Image capture systems are used in a variety of applications. Such systems may be combined in an all-in-one unit that includes image capture, image formation, and facsimile functions. Image capture systems may also be independent and only capable of performing an image capture function. Digital photograph technology has progressed with imaging units now able to produce high quality two dimensional images in a single step process. The imaging units are also economical allowing them to be utilized in new applications, such as document scanning technology.
Image capture systems having a folded optical path with a mirror have been recognized for advantageously compressing the volume of the system. Folding the optical path is seen to induce secondary reflections, e.g., a lower intensity optical ghost image of the document superimposed over a portion of the image/document captured by the sensor of the capture system. A secondary reflection is inherent in a folded optical system where the document is separated from the optical components (camera sensor, lenses, mirror, illumination, etc.) by a reflective component, such as a sheet of glass. Specifically, secondary reflections occur when there exists an optical path in the image capture system for light to leave some portion of the document, bounce first off the mirror, then back off the reflective surface at the document (scan glass), and finally off the mirror again before entering the camera sensor.
A number of attempts have been made to address the capture of secondary reflections. First, the scan glass, i.e., the platen against which the document to be scanned is disposed, was implemented as anti-reflective glass. The anti-reflective coating on the scan platen was formulated to return 1% or less of the RGB wavelengths used to illuminate the document. However, this approach proved to be insufficient to completely eliminate secondary reflections. Further, once gamma correction was applied to the resulting image, the undesirable reflection was further enhanced.
Next, image processing algorithms were investigated to remove the secondary reflection. However, algorithms were unable to distinguish between primary and secondary reflections in the captured image of the document being scanned. For example, if a white area reflected over a black region, the reflection would appear gray. No method was successfully devised that would distinguish between the resulting grey region and an equivalent gray nearby in the document. In addition, if the reflection pushed a region into saturation, it is impossible to determine the true intensity of the region once the reflection was removed.
Accordingly, it will be appreciated that an image capture system having a folded optical path that substantially reduces or eliminates the occurrence of secondary reflections is desired.